Allow pre-1970 dates in If-[Un]Modified-Since

If I want to fetch an object only if it is newer than the first moon
landing, I send a GET request with header:

    If-Modified-Since: Sun, 20 Jul 1969 20:18:00 UTC

Since that date is older than Swift, I expect a 2xx response. However,
I get a 412, which isn't even a valid thing to do for
If-Modified-Since; it should either be 2xx or 304. This is because of
two problems:

(a) Swift treats pre-1970 dates as invalid, and

(b) Swift returns 412 when a date is invalid instead of ignoring it.

This commit makes it so any time between datetime.datetime.min and
datetime.datetime.max is an acceptable value for If-Modified-Since and
If-Unmodified-Since. Dates outside that date range are treated as
invalid headers and thus are ignored, as RFC 2616 section 14.28
requires ("If the specified date is invalid, the header is ignored").

This only works for dates that the Python standard library can parse,
which on my machine is 01 Jan 1 to 31 Dec 9999. Eliminating those
restrictions would require implementing our own date parsing and
comparison, and that's almost certainly not worth it.

Change-Id: I4cb4903c4e5e3b6b3c9506c2cabbfbda62e82f35

swift/common/swob.py

@@ -138,12 +138,9 @@ def _datetime_property(header):
         if value is not None:
             try:
                 parts = parsedate(self.headers[header])[:7]
-                date = datetime(*(parts + (UTC,)))
+                return datetime(*(parts + (UTC,)))
             except Exception:
                 return None
-            if date.year < 1970:
-                raise ValueError('Somehow an invalid year')
-            return date
 
     def setter(self, value):
         if isinstance(value, (float, int, long)):

swift/obj/server.py

@@ -481,21 +481,16 @@ class ObjectController(object):
                 obj_size = int(metadata['Content-Length'])
                 file_x_ts = metadata['X-Timestamp']
                 file_x_ts_flt = float(file_x_ts)
-                try:
-                    if_unmodified_since = request.if_unmodified_since
-                except (OverflowError, ValueError):
-                    # catches timestamps before the epoch
-                    return HTTPPreconditionFailed(request=request)
                 file_x_ts_utc = datetime.fromtimestamp(file_x_ts_flt, UTC)
+
+                if_unmodified_since = request.if_unmodified_since
                 if if_unmodified_since and file_x_ts_utc > if_unmodified_since:
                     return HTTPPreconditionFailed(request=request)
-                try:
-                    if_modified_since = request.if_modified_since
-                except (OverflowError, ValueError):
-                    # catches timestamps before the epoch
-                    return HTTPPreconditionFailed(request=request)
+
+                if_modified_since = request.if_modified_since
                 if if_modified_since and file_x_ts_utc <= if_modified_since:
                     return HTTPNotModified(request=request)
+
                 keep_cache = (self.keep_cache_private or
                               ('X-Auth-Token' not in request.headers and
                                'X-Storage-Token' not in request.headers))

test/unit/common/test_swob.py

@@ -18,6 +18,7 @@
 import unittest
 import datetime
 import re
+import time
 from StringIO import StringIO
 from urllib import quote
 
@@ -644,13 +645,18 @@ class TestRequest(unittest.TestCase):
         self.assertEquals(req.headers['If-Unmodified-Since'], 'something')
         self.assertEquals(req.if_unmodified_since, None)
 
-        req.if_unmodified_since = -1
-        self.assertRaises(ValueError, lambda: req.if_unmodified_since)
-
         self.assert_('If-Unmodified-Since' in req.headers)
         req.if_unmodified_since = None
         self.assert_('If-Unmodified-Since' not in req.headers)
 
+        too_big_date_list = list(datetime.datetime.max.timetuple())
+        too_big_date_list[0] += 1  # bump up the year
+        too_big_date = time.strftime(
+            "%a, %d %b %Y %H:%M:%S UTC", time.struct_time(too_big_date_list))
+
+        req.if_unmodified_since = too_big_date
+        self.assertEqual(req.if_unmodified_since, None)
+
     def test_bad_range(self):
         req = swift.common.swob.Request.blank('/hi/there', body='hi')
         req.range = 'bad range'

test/unit/obj/test_server.py

@@ -17,6 +17,7 @@
 """Tests for swift.obj.server"""
 
 import cPickle as pickle
+import datetime
 import operator
 import os
 import mock
@@ -24,7 +25,7 @@ import unittest
 import math
 from shutil import rmtree
 from StringIO import StringIO
-from time import gmtime, strftime, time
+from time import gmtime, strftime, time, struct_time
 from tempfile import mkdtemp
 from hashlib import md5
 
@@ -1911,16 +1912,16 @@ class TestObjectController(unittest.TestCase):
             headers={'If-Modified-Since': 'Not a valid date'})
         resp = req.get_response(self.object_controller)
         self.assertEquals(resp.status_int, 200)
+
+        too_big_date_list = list(datetime.datetime.max.timetuple())
+        too_big_date_list[0] += 1  # bump up the year
+        too_big_date = strftime(
+            "%a, %d %b %Y %H:%M:%S UTC", struct_time(too_big_date_list))
         req = Request.blank(
             '/sda1/p/a/c/o', environ={'REQUEST_METHOD': 'GET'},
-            headers={'If-Unmodified-Since': 'Sat, 29 Oct 1000 19:43:31 GMT'})
+            headers={'If-Unmodified-Since': too_big_date})
         resp = req.get_response(self.object_controller)
-        self.assertEquals(resp.status_int, 412)
-        req = Request.blank(
-            '/sda1/p/a/c/o', environ={'REQUEST_METHOD': 'GET'},
-            headers={'If-Modified-Since': 'Sat, 29 Oct 1000 19:43:31 GMT'})
-        resp = req.get_response(self.object_controller)
-        self.assertEquals(resp.status_int, 412)
+        self.assertEquals(resp.status_int, 200)
 
     def test_content_encoding(self):
         req = Request.blank(